1. Field of the Invention
The present invention relates to a method for manufacturing a SIMOX wafer based on a SIMOX (Separation by IMplanted OXygen) technology among methods for manufacturing an SOI (Silicon On Insulater) wafer in which a single crystal silicon layer is formed in a silicon single crystal main body via an oxide film and a SIMOX wafer obtained by use of the method.
2. Description of the Related Art
Conventionally, two methods have been known as a method for manufacturing an SOI wafer, one is a bonding method in which two wafers are bonded with each other via an oxide film, and the other is a SIMOX method in which oxygen ions (O+) are implanted from a surface of the silicon wafer to form an ion-implanted layer of a single crystal containing highly concentrated oxygen in a region of a predetermined depth within the wafer, and then this wafer is subjected to a heat treatment to thereby transform the ion-implanted layer into a buried oxide film (Buried OXide: hereinafter, referred to as BOX layer.). An SOI wafer manufactured by the SIMOX method is particularly called a SIMOX wafer.
The method for manufacturing the SIMOX wafer at an initial development stage has been based on a high dose technology. This method for manufacturing the high dose SIMOX wafer is such that oxygen atoms are implanted into the silicon wafer at the rate of about 2×1018 atoms/cm2 with an implantation energy of about 200 keV, the BOX layer is then formed within the wafer while keeping the wafer ion-implanted (as-implanted), and then the wafer is subjected to a high temperature annealing treatment. By this annealing treatment, a crystal defect generated in the SOI layer can be recovered, and the BOX layer can be transformed.
The SIMOX wafer obtained by this high dose technology, however, has a high dose amount of oxygen ions, so that there have been problems that many threading dislocations occur in the SOI layer, a long ion implantation time is required to thereby reduce manufacturing efficiency. This threading dislocation leads to a leakage current and deterioration in a heterointerface when a device is manufactured, so that there is a possibility of preventing improvement in device performance and performance of functionality.
For that reason, in order to achieve a reduction in threading dislocation generated in the SOI layer and a shortening of the ion implantation time, a low dose implantation technology has been developed. A method for manufacturing this low dose SIMOX wafer is such that oxygen atoms are implanted from a surface of the silicon wafer at the rate of about 3.5×1017 to 4.5×1017 atoms/cm2 with an implantation energy of about 180 keV, and then the BOX layer is formed continuously in a plane direction parallel to the surface of the silicon wafer by performing a high temperature heat treatment. In a case where implantation energy is 180 keV, it is possible to form the BOX layer continuously in the plane direction parallel to the wafer surface only when the dose amount is 3.5×1017 to 4.5×1017 atoms/cm2. A range of a width of this dose amount is called a dose window. The SIMOX wafer obtained by this low dose implantation technology can reduce a threading dislocation density in the SOI layer, and can improve manufacturing efficiency due to the shortening of the ion implantation time.
However, since the SIMOX wafer obtained by use of this low dose implantation technology has a small oxygen ion dose amount, a film thickness of the BOX layer becomes thin, thereby there has been a problem in reliability of the BOX layer. In addition, when the film thickness of the formed BOX layer is thin, if a particle adheres to a surface of the silicon wafer during the ion implantation, this particle serves as a mask, so that an unimplantable portion is easily generated in the ion-implanted layer formed within the silicon wafer. While the ion-implanted layer turns into the BOX layer by annealing treatment, the unimplantable portion becomes a pinhole that is one kind of crystal defect of the BOX layer, thus degrading electrical insulation characteristics. There has been a problem that the percentage of this pinhole density is higher than that of a high dose SIMOX wafer, or the like.
Therefore, in order to inhibit the pinhole generation in the BOX layer, there is proposed a method for manufacturing an SOI substrate called an ITOX (Internal Thermal OXidation) technology, in which the ion implanted silicon wafer is subjected to annealing treatment, and then it is further subjected to an oxidizing treatment in a high temperature oxygen atmosphere, and an SOI substrate manufactured by the method (See Japanese Unexamined Patent Application Publication No. H07-263538 (claim 1, claim 6, paragraph [0009], [0010], [0025], [0026], FIG. 1)).
According to the method disclosed in Japanese Unexamined Patent Application Publication No. H07-263538, first, oxygen ions are implanted from the surface of the silicon wafer at the rate of about 4×1017 atoms/cm2 with the implantation energy of about 180 keV. Next, the wafer in which the ion-implanted layer is formed is subjected to annealing treatment in an inert gas atmosphere containing oxygen of less than 1% concentration to transform the ion-implanted layer into the BOX layer, and then this wafer is further subjected to a high-temperature oxidizing treatment in an atmosphere containing highly concentrated oxygen more than 1% concentration. When the wafer is subjected to the high-temperature oxidizing treatment, highly concentrated oxygen in the atmosphere enters the wafer from front and rear surfaces of the wafer to be diffused therein. When this oxygen stays in a boundary surface of the BOX layer as SiO2 to be stacked, the BOX layer grows, thus allowing an increase in film thickness of the BOX layer. In an SIMOX wafer obtained by this ITOX technology, when SiO2 is stacked in the interface portion of the BOX layer, the pinholes generated in the BOX layer are remedied, resulting in a reduction in the pinhole density. Furthermore, roughness of an interface between the BOX layer and the silicon wafer can also be improved. As a result, electrical characteristics of the device can be homogenized.
Even in the manufacturing method based on this ITOX technology, however, since an oxygen ion dose amount is as large as 4×1017 atoms/cm2, it takes a long time to implant the ions and the high-temperature oxidizing treatment is also needed in addition to the annealing treatment, so that there have been problems that manufacturing efficiency is low and productivity is decreased.
Thus, in order to shorten the ion implantation time, there has been proposed a method for manufacturing the SOI wafer called a MLD (Modified Low Dose) technology where oxidizing treatment is applied to a wafer having two-implanted layers including an amorphous layer formed therein (See U.S. Patent Publication No. 5930643 (claim 1, specification page 1, right second column 5th to 43rd lines, and FIG. 1(a))).
The method disclosed in U.S. Patent Publication No. 5930643 is such that the ion implantation process is divided into two steps, and oxygen ions are implanted into the wafer at different temperatures for respective steps, so that two ion-implanted layers, namely, a single crystal layer containing highly concentrated oxygen and an amorphous layer having different crystal states are formed within the wafer, and then this wafer is subjected to a high-temperature oxidizing treatment in a mixed gas atmosphere.
Specifically, firstly, in a state where the silicon wafer is heated, oxygen ions are implanted at the rate of 2×1017 atoms/cm2 with the implantation energy of 185 keV to form a first ion-implanted layer containing highly concentrated oxygen within the wafer. Next, in a state where this wafer is cooled, oxygen ions are implanted at the rate of 3×1014 atoms/cm2 with the implantation energy of 185 keV to form a second ion-implanted amorphous layer continuously to a plane on a wafer surface side of the first ion-implanted layer. A lower portion of the second ion-implanted layer overlaps with an upper portion of the first ion-implanted layer containing highly concentrated oxygen, so that highly concentrated oxygen is contained within the second ion-implanted amorphous layer. This wafer is further heated in an inert atmosphere, such as argon containing 0.5 to 5% oxygen, and then it is held at a high temperature in a 5 to 100% oxygen atmosphere containing argon or the like. The first ion-implanted layer turns into the BOX layer by holding the wafer at the high temperature in this atmosphere containing oxygen.
Meanwhile, since the second ion-implanted layer contains highly concentrated oxygen within the ion-implanted amorphous layer, re-crystallization does not proceed smoothly during the temperature rise, resulting in a high-density defect layer including a polycrystal, a twin crystal, or a stacking fault. Since a region having this defect layer formed has a property that oxygen easily precipitates therein, oxygen in the atmosphere containing oxygen enters the wafer from front and rear surfaces of the wafer to diffuse within the wafer during holding the wafer at a constant temperature after the temperature rise, and oxygen is concentrated in this high-density defect layer to be precipitated as SiO2. Thus, the high-density defect layer having SiO2 precipitated therein turns into the BOX layer. For this reason, a low ion dose amount of 2×1017 atoms/cm2 allows to obtain a SIMOX wafer having the BOX layer with the same thickness as that obtained when a double dose amount is implanted.